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<tr class="t-nv"><td colspan="5"> <a href="statements.html" title="cpp/language/statements"> <span class="t-spar">label</span> : <span class="t-spar">statement</span></a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="statements.html" title="cpp/language/statements"> <span class="t-spar">expression</span> ; </a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="statements.html" title="cpp/language/statements"> { <span class="t-spar">statement</span>... } </a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="if.html" title="cpp/language/if"> if</a></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> switch</strong></td></tr>
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</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>根据条件的值，将控制流转移到若干语句之一。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(可选)</span> <code><b>switch</b></code> <code><b>(</b></code> <span class="t-spar">条件</span> <code><b>)</b></code> <span class="t-spar">语句</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td> <span class="t-mark-rev t-until-cxx17">(C++17 前)</span>
</td></tr>




<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(可选)</span> <code><b>switch</b></code> <code><b>(</b></code> <span class="t-spar">初始化语句</span><span class="t-mark">(可选)</span> <span class="t-spar">条件</span> <code><b>)</b></code> <span class="t-spar">语句</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td> <span class="t-mark-rev t-since-cxx17">(C++17 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td>  任意数量的<a href="attributes.html" title="cpp/language/attributes">属性</a>
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">条件</span>
</td>
<td> -
</td>
<td>  任何整型或枚举类型，或<a href="implicit_cast.html" title="cpp/language/implicit cast" class="mw-redirect">可按语境隐式转换</a>到整型或枚举类型的类类型的<a href="expressions.html" title="cpp/language/expressions">表达式</a>，或单个这种类型的非数组变量的，带花括号或等号<a href="initialization.html" title="cpp/language/initialization">初始化器</a>的声明。
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">初始化语句</span><span class="t-mark-rev t-since-cxx17">(C++17)</span>
</td>
<td> -
</td>
<td>  下列之一
<ul><li> 一条<a href="statements.html#.E8.A1.A8.E8.BE.BE.E5.BC.8F.E8.AF.AD.E5.8F.A5" title="cpp/language/statements">表达式语句</a>（可为<i>空语句</i>“<code><b>;</b></code>”）
</li><li> 一条<a href="declarations.html" title="cpp/language/declarations">简单声明</a>，典型地为带初始化器的变量声明，但它可以声明任意多个变量或结构化绑定
</li></ul>
<dl><dd>注意任何 <span class="t-spar">初始化语句</span> 必须以分号 <code><b>;</b></code> 结尾，这就是为什么它常被非正式描述为后随分号的表达式或声明。
</dd></dl>
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">语句</span>
</td>
<td> -
</td>
<td>  任何<a href="statements.html" title="cpp/language/statements">语句</a>（典型地为复合语句）。在 <span class="t-spar">语句</span> 中允许使用 <code><b>case:</b></code> 和 <code><b>default:</b></code> 标号，且 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">break</span><span class="sy4">;</span></span></span> 语句具有特殊含义。
</td></tr></table>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(可选)</span> <code><b>case</b></code> <span class="t-spar">常量表达式</span> <code><b>:</b></code> <span class="t-spar">语句</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(可选)</span> <code><b>default</b></code> <code><b>:</b></code> <span class="t-spar">语句</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">常量表达式</span>
</td>
<td> -
</td>
<td>  与 <span class="t-spar">条件</span> 经过转换和<a href="implicit_cast.html" title="cpp/language/implicit cast" class="mw-redirect">整型提升</a>后的类型相同的常量表达式
</td></tr></table>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p>switch 语句的体可拥有任意数量的 <code>case:</code> 标号，只要所有 <span class="t-spar">常量表达式</span> 的值（转换/提升后）均为唯一。至多可以存在一个 <code>default:</code> 标号（尽管嵌套的 switch 语句可使用其自身的 <code>default:</code> 标号，或拥有和外部 switch 使用完全相同的常量的 <code>case:</code> 标号）
</p><p>若 <span class="t-spar">条件</span> 求值为等于 <span class="t-spar">常量表达式</span> 之一的值，则控制被转移到用该 <span class="t-spar">常量表达式</span> 标号化的语句。
</p><p>若 <span class="t-spar">条件</span> 求值为不匹配任何 <code>case:</code> 标号的值，且存在 <code>default:</code> 标号，则控制被转移到带有 <code>default:</code> 标号的语句。
</p><p>当 <span class="t-spar">语句</span> 中遇到 <a href="break.html" title="cpp/language/break">break</a> 语句时退出 switch 语句：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">switch</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">case</span> <span class="nu0">1</span> <span class="sy4">:</span> cout <span class="sy1">&lt;&lt;</span> <span class="st0">'1'</span><span class="sy4">;</span> <span class="co1">// 打印 "1",</span>
    <span class="kw1">case</span> <span class="nu0">2</span> <span class="sy4">:</span> cout <span class="sy1">&lt;&lt;</span> <span class="st0">'2'</span><span class="sy4">;</span> <span class="co1">// 然后打印 "2"</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">switch</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">case</span> <span class="nu0">1</span> <span class="sy4">:</span> cout <span class="sy1">&lt;&lt;</span> <span class="st0">'1'</span><span class="sy4">;</span> <span class="co1">// 打印 "1"</span>
             <span class="kw1">break</span><span class="sy4">;</span>       <span class="co1">// 然后退出 switch</span>
    <span class="kw1">case</span> <span class="nu0">2</span> <span class="sy4">:</span> cout <span class="sy1">&lt;&lt;</span> <span class="st0">'2'</span><span class="sy4">;</span>
             <span class="kw1">break</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p>编译器可能在发生直落（抵达下个 case 标号而无 break）时发布警告，除非属性 <a href="attributes.html" title="cpp/language/attributes"><code>[[fallthrough]]</code></a> 紧接该标号之前出现以指示其直落是故意如此。
</p><p>若使用 <span class="t-spar">初始化语句</span>，则 switch 语句等价于
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>{</b></code><br>
<dl><dd><span class="t-spar">初始化语句</span> <br>
</dd><dd><code><b>switch</b></code> <code><b>(</b></code> <span class="t-spar">条件</span> <code><b>)</b></code> <span class="t-spar">语句</span> <br>
</dd></dl>
<p><code><b>} </b></code>
</p>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>但 <span class="t-spar">初始化语句</span> 所声明的名字（若 <span class="t-spar">初始化语句</span> 是声明）和 <span class="t-spar">条件</span> 所声明的名字（若条件是声明）处于同一作用域中，该作用域亦是 <span class="t-spar">语句</span> 的作用域。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr></table>
<p>因为控制转移时<a href="goto.html" title="cpp/language/goto">不允许进入变量的作用域</a>，所以若在 <span class="t-spar">语句</span> 中遇到声明语句，则它的作用域必须被限制在其自身的复合语句之内：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">switch</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">case</span> <span class="nu0">1</span><span class="sy4">:</span> <span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// 初始化</span>
            <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> x <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
            <span class="kw1">break</span><span class="sy4">;</span>
    <span class="kw1">default</span><span class="sy4">:</span> <span class="co1">// 编译错误：跳到 default: 会进入 'x' 的作用域而不初始化它</span>
             <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"default<span class="es1">\n</span>"</span><span class="sy4">;</span>
             <span class="kw1">break</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">switch</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">case</span> <span class="nu0">1</span><span class="sy4">:</span> <span class="br0">{</span>  <span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
               <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> x <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
               <span class="kw1">break</span><span class="sy4">;</span>
            <span class="br0">}</span> <span class="co1">// 'x' 的作用域在此结束</span>
    <span class="kw1">default</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"default<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// 无错误</span>
             <span class="kw1">break</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
<p><a href="../keyword/switch.html" title="cpp/keyword/switch"><tt>switch</tt></a>,
<a href="../keyword/case.html" title="cpp/keyword/case"><tt>case</tt></a>,
<a href="../keyword/default.html" title="cpp/keyword/default"><tt>default</tt></a>
</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><p>下列代码展示 <i>switch</i> 语句的几种用法</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
    <span class="kw1">switch</span> <span class="br0">(</span>i<span class="br0">)</span> <span class="br0">{</span>
        <span class="kw1">case</span> <span class="nu0">1</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"1"</span><span class="sy4">;</span>
        <span class="kw1">case</span> <span class="nu0">2</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"2"</span><span class="sy4">;</span>   <span class="co1">//执行始于此标号</span>
        <span class="kw1">case</span> <span class="nu0">3</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"3"</span><span class="sy4">;</span>
        <span class="kw1">case</span> <span class="nu0">4</span><span class="sy4">:</span>
        <span class="kw1">case</span> <span class="nu0">5</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"45"</span><span class="sy4">;</span>
                <span class="kw1">break</span><span class="sy4">;</span>              <span class="co1">//终止后继语句的执行</span>
        <span class="kw1">case</span> <span class="nu0">6</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"6"</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    <span class="kw1">switch</span> <span class="br0">(</span>i<span class="br0">)</span> <span class="br0">{</span>
        <span class="kw1">case</span> <span class="nu0">4</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"a"</span><span class="sy4">;</span>
        <span class="kw1">default</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"d"</span><span class="sy4">;</span> <span class="co1">//无适用的常量表达式</span>
                                   <span class="co1">//故执行 default</span>
    <span class="br0">}</span>
 
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    <span class="kw1">switch</span> <span class="br0">(</span>i<span class="br0">)</span> <span class="br0">{</span>
        <span class="kw1">case</span> <span class="nu0">4</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"a"</span><span class="sy4">;</span>  <span class="co1">//未执行任何语句</span>
    <span class="br0">}</span>
 
    <span class="co1">// 当在 switch 语句中使用枚举时，若枚举项之一未处理，许多编译器都会发布警告</span>
    <span class="kw2">enum</span> color <span class="br0">{</span>RED, GREEN, BLUE<span class="br0">}</span><span class="sy4">;</span>
    <span class="kw1">switch</span><span class="br0">(</span>RED<span class="br0">)</span> <span class="br0">{</span>
        <span class="kw1">case</span> RED<span class="sy4">:</span>   <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"red<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
        <span class="kw1">case</span> GREEN<span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"green<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
        <span class="kw1">case</span> BLUE<span class="sy4">:</span>  <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"blue<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <span class="co1">// C++17 的 初始化语句 语法在不存在到整型或枚举类型的隐式转换时很有用</span>
    <span class="kw1">switch</span> <span class="br0">(</span>Device dev <span class="sy1">=</span> get_device<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> dev.<span class="me1">state</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>
    <span class="br0">{</span>
       <span class="kw1">case</span> SLEEP<span class="sy4">:</span> <span class="coMULTI">/*...*/</span> <span class="kw1">break</span><span class="sy4">;</span>
       <span class="kw1">case</span> READY<span class="sy4">:</span> <span class="coMULTI">/*...*/</span> <span class="kw1">break</span><span class="sy4">;</span>
       <span class="kw1">case</span> BAD<span class="sy4">:</span> <span class="coMULTI">/*...*/</span> <span class="kw1">break</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <span class="co1">// 有问题的示例</span>
 
    <span class="co1">// 语句不必是复合语句</span>
    <span class="kw1">switch</span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span>
        <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"什么也不会发生<span class="es1">\n</span>"</span><span class="sy4">;</span>
 
    <span class="co1">// 标号也不要求复合语句</span>
    <span class="kw1">switch</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="br0">)</span>
        <span class="kw1">case</span> <span class="nu0">0</span><span class="sy4">:</span>
        <span class="kw1">case</span> <span class="nu0">1</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> n <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    <span class="co1">// 达夫设备： http://en.wikipedia.org/wiki/Duff's_device</span>
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">2345
d
red
1</pre></div></div> 
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

</table>

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